<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>*{
  margin: 0;
  padding: 0;
}

body {
  width: calc(100vw - 200px);
  height:calc(100vh - 300px);
  margin: 0 auto;
  background: red;
}</style>
</head>

<body>
  <script>class Person {
  constructor(data) {
    this.data = data;
  }
  say() {
    return 'hello world'
  }
}

const person = new Person()
person.say()</script>
</body>

</html>